The two key factors for the generation of the diverse antigen receptor repertoire in the human adaptive immune system are the products of the recombination activating genes 1 and 2 (RAG1 and RAG2). The identification of their homologs (spRAG1L and spRAG2L, respectively) in the genome of the purple sea urchin (Strongylocentrotus purpuratus) was unexpected as there is no evidence thus far of an adaptive immune system in invertebrates (including echinoderms), i.e. they lack at least one of its hallmarks, a diversified antigen receptor repertoire. During this fiscal year we established a collaboration with Dr. Barreo's lab in Portugal to test tehe activity of sea urchin RAG1/2 and mouse/urchin hybrid proteins using a novel sensitive fluorescence recombination reporter. We also established the protocols and procedure for a complete next-generation sequencing approach to determine the transcriptome of coelomocyte subpopulations. Our studies have major implications for the current model of how adaptive immunity evolved in jawed vertebrates, and will help to illuminate conserved features of how V(D)J recombination is tightly controlled to avoid potentially dangerous modifications of the genome.